Overview

In AsyncResultsMerger::_handleBatchResponse(), _keepRecordOfAdditionalTxnParticipants() is called before the lifecycle/closed checks and before the try block that guards the rest of response handling. The helper runs TransactionRouter::Router::parseParticipantResponseMetadata(), which strictly parses TxnResponseMetadata and throws on fields that are present but badly typed.

Impact

If a shard returns a getMore reply with malformed transaction fields, the parse throws before the remote is woken (_signalCurrentEventIfReady()), before _cleanUpKilledBatch(), and before batch processing, stalling the router-side cursor (or letting the error escape the executor callback). Availability only; requires control of a server in the cluster. Pre-existing: the base tree had the same inline parse at the same position.

Scope

* Move the _keepRecordOfAdditionalTxnParticipants() call inside the existing try block (or otherwise handle a parse failure) so the wake-up/cleanup paths still run.
* Add a unit test covering a getMore reply with malformed transaction metadata.

Notes

Split out of SERVER-137535 (PR #64898); tracked as an 8th PR in the ARM refactor stack.